NHAMCS Estimation Procedures Statistics from the NHAMCS are derived by a multistage estimation procedure that produces essentially unbiased estimates. The estimation procedure has three basic components: 1) inflation by reciprocals of the sampling selection probabilities; 2) adjustment for nonresponse; and 3) a population weighting ratio adjustment. Beginning with 1997 data, the population weighting ratio adjustment for OPD estimates was replaced by an adjustment that controls for effects of rotating hospital sample panels into and out of the sample each year. (The full NHAMCS hospital sample is partitioned into 16 panels which are rotated into the sample over 16 periods of 4 weeks each so that only 13 panels are used in each year.) 1. Inflation by
reciprocals of selection probabilities 2. Adjustment for
nonresponse The second type of nonresponse occurred when a sample ESA or OPD clinic within a "respondent" hospital failed to provide completed Patient Record forms for a sample of its patient visits. The weights of visits to ESAs/OPD clinics similar to the nonrespondent ESAs/OPD clinics were inflated to account for visits represented by the nonrespondent ESAs/OPD clinics where ESAs/OPD clinics were judged to be similar if they were in the same region, ownership control group, MSA status group, and ESA/OPD clinic group. For this purpose, there were six OPD clinic groups: general medicine, pediatrics, surgery, OB/GYN, alcohol and/or substance abuse, and "other." Beginning with 2004 data, changes were made to the nonresponse adjustment factor to account for the seasonality of the reporting period. Extra weights for nonresponding hospital outpatient departments and emergency departments were shifted to responding outpatient and emergency departments in reporting periods within the same quarter of the year. The shift in nonresponse adjustment did not significantly affect any of the overall annual estimates. 3. Ratio adjustments Beginning with the 1997 NHAMCS, the adjustment for OPD estimates was replaced by a ratio that had as its numerator the weighted OPD visit volumes of hospitals in the full NHAMCS sample (16 hospital panels) and as its denominator the weighted OPD visit volumes of hospitals in the 13 hospital panels included in that year’s sample. This adjustment used visit volumes that were based on the most recent survey data collected from hospitals that had participated in NHAMCS for at least one year. For hospitals that had never participated, visit volumes were obtained by phone, from SMG data, or by using the average of visit volumes for refusal hospitals which had converted to respondent status in the 1998 survey.
This page last reviewed
January 11, 2007
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